1. Field of the Invention
The present invention relates to a laser processing device having a function for starting laser processing of an object to be processed, while reducing a reflected laser beam from the object.
2. Description of the Related Art
In some prior art techniques, when laser processing is carried out by irradiating a laser beam onto an object (or a workpiece) such as a metal material to be processed, a reflected laser beam from the workpiece is used to improve welding quality. For example, JP 2706498 B discloses a laser processing device including: a reflected beam detection means for detecting a level of a laser beam reflected by a workpiece surface and returned into a laser oscillator; a sense means for sensing that the level of the reflected beam falls below a predetermined threshold; and a command means for starting a piercing process by irradiating a laser beam at a required output level, terminating the piercing process when the level of the reflected beam falls below the threshold, and subsequently outputting a command for the next piercing process.
JP 2012-076088 A discloses a method, in which, prior to a laser cutting process of a plate-like workpiece, piercing processing is carried out multiple times by variously changing an approach position of a light condensing lens relative to a workpiece so as to variously change a focal position for the workpiece, and then a laser cutting process of the workpiece is carried out while maintaining the focal position where a detection value of an amount of scattering light detected in the multiple times of piercing is minimum.
If an object to be processed is a metal material such as steel or aluminum, which easily reflects a laser beam, at the moment when a laser beam is irradiated onto the workpiece, a part of the irradiated laser beam is reflected so as to reversely progress along a path similar to the outward path, and then the part of the laser beam is returned to a laser oscillator as the reflected beam, whereby the laser oscillator or a beam path may be damaged. As such, due to the reflected laser beam returned to the laser oscillator, the laser power cannot be controlled and/or the optical system may be damaged. As a relevant prior art document for avoiding such a problem, JP S62-289387 A discloses a technique to incline at least one of an irradiation head and a reflecting material, so that an optical axis of an irradiating laser beam is not aligned with an optical axis of a reflected laser beam.
As described above, in case that the object to be processed is a metal material such as steel or aluminum, by which the laser beam is easily to be reflected, or that an energy density at a processing point is relatively low, when a light condensing point is positioned on a surface of the object, the part of the irradiated laser beam may reversely progress along the path similar to the outward path so as to be returned to the laser oscillator as the reflected beam. As the amount (or intensity) of the reflected beam is high, the laser light source or the beam path of the laser processing device may be increased. In the prior art, in order to avoid the excess reflected beam, it is necessary to irradiate the laser beam onto the surface of the object at a tentative processing condition, and then change (or optimize) the processing condition so as to reduce the intensity of the reflected beam, when the intensity of the reflected beam is high enough to create an adverse effect.
Although the technique of JP 2706498 B or JP 2012-076088 A is intended to detect the reflected beam (or the scattering light), the technique is not intended to reduce or eliminate the adverse effect of the reflected beam on the laser light source, etc. On the other hand, in JP S62-289387 A, at least one of the irradiation head and the reflecting material is inclined so as to avoid the adverse effect of the reflected beam. However, when the laser beam is obliquely irradiated to the surface of the object, the quality of the laser processing may be deteriorated, compared to a case in which the laser beam is generally vertically irradiated to the object. Otherwise, in the configuration of JP S62-289387 A, it is necessary to arrange a means for arbitrarily change an angle of the object relative to the laser beam, in order to vertically irradiate the laser beam onto the object.